A biaxially oriented polyester film is used as the base of a magnetic tape or a flexible printed circuit board.
However, to obtain a labor-saving effect at the processing step in the production of a magnetic tape or a flexible printed circuit board, or with an increase of the height of the application object, the demand for a film having an enhanced elastic modulus and a further improved dimensional stability is increasing. However, although the elastic modulus of a biaxially oriented polyester film is increased by drawing in multiple stages in the film-forming process, the dimensional stability is degraded by this multiple-stage drawing. Attempts to improve the mechanical characteristics of the biaxially drawn polyester film by making the film from a blend of a polymer having a rigid polymer chain, such as an aromatic polyester having a melt anisotropy-forming capacity, with a polymer having a flexible polymer chain have been made. However, the mechanical characteristics are not improved because of an insufficient dispersion of the rigid polymer, and because the rigid polymer peels from the interface of the dispersed phase. Utilization can be made only in a product obtained by drawing a fiber made of a blend of a rigid chain polymer with a polyester to form voids in the interface of the dispersed phase, and thereby mat the film surface.
It is an object of the present invention to provide a polyester film having a high elastic modulus, a low thermal shrinkage and an excellent dimensional stability, in which the defects of the conventional biaxially oriented polyester film are overcome by finely dispersing a polymer having a rigid molecule chain in a polyester.